1. CMPT 365 Multimedia Systems
Media Compression - Image Spring 2015

2. Facts about JPEG JPEG - Joint Photographic Experts Group
International standard: 1992
Most popular format
Other formats (.bmp) use similar techniques
Lossy image compression
transform coding using the DCT
JPEG 2000
New generation of JPEG – well, never succeeds
DWT (Discrete Wavelet Transform)

3. Three Major Observations
Useful image contents change relatively slowly across the image, i.e., it is unusual for intensity values to vary widely several times in a small area, for example, within an 8x8 image block.
- much of the information in an image is repeated, hence “spatial redundancy".
Compression Ratio: 7.7
Compression Ratio: 33.9

4. Observations Observation 2:
Psychophysical experiments suggest that humans are much less likely to notice the loss of very high spatial frequency components than the loss of lower frequency components.
- the spatial redundancy can be reduced by largely reducing the high spatial frequency contents.
Compression Ratio: 7.7
Compression Ratio: 33.9

5. Observations Observation 3:
Visual acuity (accuracy in distinguishing closely spaced lines) is much greater for gray (black and white) than for color.
- chroma subsampling (4:2:0) is used in JPEG.

6. 8x8 DCT Example or u Original values of an 8x8 block
or v
or u
DC Component
Original values of an 8x8 block
(in spatial domain)
Corresponding DCT coefficients
(in frequency domain)

7. JPEG Steps Block Preparation Transform Quantization
- RGB to YUV (YIQ) planes
Transform
- 2D Discrete Cosine Transform (DCT) on 8x8 blocks.
Quantization
- Quantized DCT Coefficients (lossy).
Encoding of Quantized Coefficients
Zigzag Scan
Differential Pulse Code Modulation (DPCM) on DC component
Run Length Encoding (RLE) on AC Components
Entropy Coding: Huffman or Arithmetic

8. JPEG Diagram

9. JPEG: Block Preparation
RGB Input Data
After Block Preparation
Input image: 640 x 480 RGB (24 bits/pixel) transformed to three planes:
Y: (640 x 480, 8-bit/pixel) Luminance (brightness) plane.
U, V: (320 X bits/pixel) Chrominance (color) planes.

10. Block Effect
Using blocks, however, has the effect of isolating each block from its neighboring context.
choppy (“blocky") with high compression ratio
Compression Ratio: 7.7
Compression Ratio: 33.9
Compression Ratio: 60.1

11. JPEG: Quantized DCT Coefficients
q(u,v)
Uniform quantization:
Divide by constant N and round result.
In JPEG, each DCT F[u,v] is divided by
a constant q(u,v).
The table of q(u,v) is called quantization table.
F[u,v]
Rounded
F[u,v]/ Q(u,v)

12. More about Quantization
Quantization is the main source for loss
Q(u, v) of larger values towards lower right corner
More loss at the higher spatial frequencies
Supported by Observations 1 and 2.
Q(u,v) obtained from psychophysical studies
maximizing the compression ratio while minimizing perceptual losses

14. JPEG: Encoding of Quantized DCT Coefficients
DC Components (zero frequency)
DC component of a block is large and varied, but often close to the DC value of the previous block.
Encode the difference from previous
Differential Pulse Code Modulation (DPCM).
AC components:
Lots of zeros (or close to zero)
Run Length Encoding (RLE, or RLC)
encode as (skip, value) pairs
Skip: number of zeros, value: next non-zero component
(0,0) as end-of-block value.

15. JPEG: Zigzag Scan Maps an 8x8 block into a 1 x 64 vector
Zigzag pattern group low frequency coefficients in top of vector.

16. Recall: 2-D DCT Basis Matrices
For 2-point DCT For 4-point DCT

17. Recall: 2-D DCT Basis Matrices: 8-point DCT

18. Why ZigZag Scan RLC aims to turn the block values into sets
<#-zeros-to-skip , next non-zero value>.
ZigZag scan is more effective

19. Runlength Encoding (RLE)
A typical 8x8 block of quantized DCT coefficients.
Most of the higher order coefficients have been quantized to 0.   
12  34   0   54    0    0    0    0
87   0   0   12    3    0    0    0
16   0   0    0    0    0    0    0
0    0    0    0    0    0    0    0
Zig-zag scan: the sequence of DCT coefficients to be transmitted:
DC coefficient (12) is sent via a separate Huffman table.
Runlength coding remaining coefficients:
34 | 87 | 16 | | | |
Further compression: statistical (entropy) coding

20. Entropy Coding
Huffman/arithmetic coding
Lossless

21. JPEG Modes Sequential Mode Progressive Mode. Hierarchical Mode.
default JPEG mode, implicitly assumed in the discussions so far. Each graylevel image or color image component is encoded in a single left-to-right, top-to-bottom scan.
Progressive Mode.
Hierarchical Mode.
Lossless Mode

22. Progressive Mode Progressive Method 1. Spectral selection
Delivers low quality versions of the image quickly, followed by higher quality passes.
Method 1. Spectral selection
- higher AC components provide detail texture information
Scan 1: Encode DC and first few AC components, e.g., AC1, AC2.
Scan 2: Encode a few more AC components, e.g., AC3, AC4, AC5.
...
Scan k: Encode the last few ACs, e.g., AC61, AC62, AC63.

23. Progressive Mode cont’d
Method 2: Successive approximation:
- Instead of gradually encoding spectral bands, all DCT coefficients are encoded simultaneously but with their most significant bits (MSBs) first
Scan 1: Encode the rst few MSBs, e.g., Bits 7, 6, 5, 4.
Scan 2: Encode a few more less signicant bits, e.g., Bit 3.
...
Scan m: Encode the least signicant bit (LSB), Bit 0.

24. Hierarchical Mode Encoding Transmission:
First, lowest resolution picture (using low-pass filter)
Then, successively higher resolutions
additional details (encoding differences)
Transmission:
transmitted in multiple passes
progressively improving quality
Similar to Progressive JPEG

25. Lossless Mode Using prediction and entropy coding
Forming a differential prediction:
A predictor combines the values of up to three neighboring pixels as the predicted value for the current pixel
Seven schemes for combination
Encoding:
The encoder compares the prediction with the actual pixel value at the position `X' and encodes the difference using entropy coding

26. 7 Predictors

27. Comparison with Other Lossless

28. JPEG Bitstream

29. JPEG 2000 JPEG 1992 JPEG 2000 .jp2 for ISO/IEC 15444-1
.jpx for extended part-2 specifications (ISO/IEC )
Wavelet transform based
20% gain in compression

30. JPEG 2000 vs JPEG
Original image

31. JPEG2000 vs JPEG

32. Further Exploration Textbook Chapter 9 Other sources
The JPEG Still Image Compression Standard by Pennebaker and Mitchell
JPEG2000: Image Compression Fundamentals, Standards, and Practice by Taubman and Marcellin
Image and Video Compression Standards: Algorithms and Architectures, 2nd ed. by Bhaskaren and Konstantinides